Rows in group of Ribbon
 

I'm making "customUI.xml file" for custom ribbon.

When assuming, ["button id="xxx" size ="normal"] , three lines are added in
the group control.

but, I want to do this to two lines.

For instance, I want to do like [group id="xxx" rows="2"].
but, In the scheme of "group contorol", there is no "rows".

In a standard ribbon of Excel, it can be achieved.

How do you achieve it in the custom ribbon?